This invention relates to a VOD refining method for fast-cut stainless steel containing sulphur, particularly to a method which improves the quality of fast-cut stainless steel by elevating the sulphur contained in the stainless steel.
What is called fast-cut stainless steel is a stainless steel containing sulphur in the scope of 0.25 wt %-0.30 wt and 0.30 wt %-0.35 wt %, which has advantages such as very glossy smoothness of its surface, ease of cutting, causing little tool wear and producing little scrap. Therefore, the tool industry needs quite a large quantity of fast-cut stainless steel.
Conventional refining processes can hardly control the quantity of sulphur contained in the fast-cut stainless steel, with the recycling percentage of sulphur being unstable, and with oxygen volume contained in fast-cut stainless steel being unstable as well (impossible to be kept in the scope of 70 ppm-100 ppm). This leads to poor quality of fast-cut stainless steel, unstable volumes of sulphur contained therein, and poor fast-cut properties, which are drawbacks yet to be resolved by the steel industry.
A tank for containing steel for refining is divided into an upper room for residue, and a lower room for steel. In order to avoid losing a large volume of sulphur in an oxygen vacuum blowing process, ferrous sulphide (or sulphur line, sulpur contained in ferrous sulphide being 29%, and sulphur line being 0.2 kg/m) is added to the steel after it produces residue, the sulphur volume being in the scope of 0.25 wt %-0.35 wt % for obtaining fast-cut steel of fine quality. But the residue phase contains calcium oxide (CaO), manganese oxide (MgO), silica (SiO2), aluminum oxide (Al2O3), which may cause the sulphur volume added to be absorbed by the steel residue thereby reducing the sulphur percent in the steel. How to elevate the sulphur volume in the steel and how to stabilize the sulphur recycling percentage are the main factors for deciding the quality of fast-cut steel.
As is known from conventional refining methods, basicity is a main parameter in refining for control of the sulphur volume or sulphur recycling percentage.
The definition of basicity is: V=CaO+MgO/SiO2+Al2O3. It is generally considered that high basicity (V greater than 1.6) can acquire better sulphur separation, and on the contrary, low basicity (V less than 1.4) stabilizes the adding effect of sulphur. Applicant has adduced in practical refining processes that the art of adding sulphur by means of low basicity residue causes low sulphur recycling percentage and unstability, resulting in inferior sulphur absorption. This inventor has found in a series of experiments that control of the low basicity residue has the following disadvantages. The related data and analysis are shown in FIGS. 1 and 2. FIG. 1 shows the relation between sulphur recycling percentage and C+M+F of a conventional refining process, with C being CaO, M being MgO, F being calcium fluoride. FIG. 2 shows graphic of change according to random time in a conventional refining process.
1. As shown in FIG. 1, aggregate sulphur recycling percentage of sulphur did not increase as expected due to low basicity. All recycling percentages of sulphur are substantially unstable and distributed substantively wide, with no rule. As shown in the distribution of recycling percentage of sulphur in the figure, recycling percentage of sulphur is low as less than 60% regardless of basicity being 1. Therefore it is hard to use low basicity as a factor in controlling the contained volume of sulphur.
2. As shown in FIG. 2, sulphur is distributed substantively unstably in phases of steel liquid and steel residue, changing with time and with difference. Thus, it is substantively difficult to control material and to adjust added volume, or to obtain an absorbing percentage of sulphur as expected.
It is expected that the low basicity residue method for controlling sulphur volume contained in fast-cut steel cannot effectively stabilize the sulphur volume absorbed or sulphur volume contained therein.
This invention has been devised to offer a VOD refining method for stabilizing the volume of sulphur absorbed and increasing the control of the sulphur volume contained in fast-cut steel.
The VOD refining method for elevating the sulphur contained in fast-cut steel includes at least the following steps.
1. VOD (Vacuum oxygen decarbonization) refining performed in a steel refining processes, with a steel liquid phase and a constant basicity residue phase formed in the steel tank;
2. a step of reducing and producing residue and adding ferrous sulfide;
3. a step of micro-adjusting control;
4. a step of removing the steel.
The feature is that 0.001 wt %-0.01 wt % CaO and 0.001 wt %-0.005 wt % MgO are added to the liquid steel in the step of micro-adjusting control, thereby elevating the sulphur absorbing percentage of the fast-cut steel and elevating aggregate recycling percent age of sulphur to more than 85%.